Odor Control 6
Natural Ventilation
Natural ventilation uses local wind and thermal bouyancy, often
called the stack effect, to move air through the structure. Fans
are not used. Instead the quantity of airflow is determined by
the size and placement of openings, roof slope, and orientation
of the building with respect to prevailing wind direction. Thermostatically
controlled side wall curtains are used in some swine buildings
to control inside temperature to some extent. However, the air
temperature of heated spaces, such as farrowing or nursery rooms,
can be controlled better with mechanical ventilation. Gestation,
breeding, and finishing barns are good candidates for natural
ventilation. Details on natural ventilation system design are
provided in the following publications:
Swine Facilities and Equipment
(AEnL 557, Cooperative Extension Service, Clemson University),
and
Natural Ventilating Systems for Livestock
Housing
(MWPS-33, Midwest Plan Service, Iowa State University).
These publications can be obtained from the Department of Agricultural
and Biological Engineering at Clemson University (864-656-3167).
The following section provides general guidelines for natural
ventilation systems.
The advantages of a natural ventilation system are: (1) reduced
operating costs as compared to mechanical systems, (2) minimal
concern of loss of ventilation due to power failure, and (3) reduced
lighting costs during the day due to increased levels of natural
light.
The disadvantages of natural ventilation are: (1) poor control
of temperature for small animals during cold weather, (2) reduced
control of air distribution within the building, and (3) lack
of effectiveness during summer conditions with minimal wind.
Site Selection
Orient the building to take advantage
of prevailing summer wind.
In South Carolina, naturally ventilated buildings should run east
and west.
Locate the building at least 50 ft from
upright silos and clusters
of trees, and 75 ft away from all other buildings. Separation
distances of 100 ft are desirable for buildings that are 16 ft
tall and 250 ft long.
If at all possible, locate the building
such that no obstructions exist on the south side of the building.
Building Characteristics And
Openings
Provide a 4/12 slope for gable or peak
roofs. Monoslope shed
roofs need a slope of at least 2/12, but 2.5/12 roofs are common.
All naturally ventilated buildings need
a sidewall height of
8 ft or more. Buildings wider than 34 ft require a 10 ft sidewall
height. A 12 ft sidewall is recommended on buildings wider than
50 ft.
Size the ridge vent according to
building width. Provide 2 inches
of continuous ridge (or peak) opening for every 10 ft of building
width. For example, a 40 ft wide building would require an 8 inch
wide ridge vent. The minimum ridge vent size is 6 inches. Do
not use a ridge cap because it will obstruct airflow. Protect
the top of the trusses with metal flashing or a short piece of
a metal ridge cap. A raised cover for the ridge vent can also
be used. However, the gap between the bottom of the cover and
the ridge vent should be the same size as the ridge vent.
Provide a 2 - 3 ft overhang along each
sidewall.
Size eave vents in a similar manner as
ridge vents. Provide
1 inch of continuous eave opening per 10 ft of building width
on each side of the building. During cold weather use manual or
automatically controlled curtains to reduce the eave vent to 1/4
to 1/2 inch per 10 ft of building width.
Provide full-wall ventilation (at least
60% of the sidewall
should be opened) in the summer. Curtains provide the easiest
and least expensive method to provide open sidewalls during summer.
